The focus of the proposed studies in the Project 3 of this Program Project is to provide high-resolution structural information, using X-ray crystallographic techniques, on both human and animal caliciviruses including the non-structural proteins to identify structural targets for the design and development of antiviral and diagnostic strategies. In aim 1, we propose to determine crystallographic structures of (1) San Miguel sea lion virus, as a representative example of an animal calicivirus in the Vesivirus genus in which the viruses show a broad host range including a potential ability to infect humans, and cause a variety of illnesses; (2) recombinant Grimsby virus capsid (rGrV), as a representative of human calicivirus in the genogroup 2 of the Norovirus genus which shows contrasting carbohydrate binding properties, and (3) recombinant Sapporo virus capsid as a representative human calicivirus in the Sapovirus genus, in which the viruses are morphologically more similar to animal caliciviruses than to human noroviruses. These studies should provide an atomic level description of the structural variations in calicivirus capsids that may form a basis for host-specificity, tissue tropism, and contrasting receptor-binding activities. These studies are particularly relevant considering the recent findings that suggest zoonotic potential and inter-species transmission of caliciviruses. In aim 2, we propose to determine the crystallographic structure of recombinant Norwalk virus (rNV) complexed with histo-blood group carbohydrates. Recently, Dr. Estes and others have shown that carbohydrates are cell-binding ligands for NV. These studies together with comparative structural analysis of rNV and rGrV should provide insights into the specificity of carbohydrate binding. The structural information learned should be useful for both the design of oligosaccharide mimics that could prevent or ameliorate infection by inhibiting virus binding to cells, and for developing carbohydrate-based diagnostic assays for human caliciviruses (Project 1 of this Program). In aim 3, we propose crystallographic studies on VP2, encoded by the third ORF of the NV genome, and two enzymes, a 3C-like protease and an NTPase encoded by the first ORF. The minor capsid protein VP2 is suggested to be involved in enhanced capsid expression and stability and genome packaging (Project 2 of this Program). Because of their specific activities during virus infection, these proteins are promising targets for antiviral drugs.